Angiosperm
Cirsium palustre in bloom, Saaremaa, Estonia fen meadow. @ C.Michael Hogan
Angiosperms
The Angiosperms, or flowering plants, described taxonomically as the division Magnoliophyta, are the largest grouping within the plant kingdom (Kingdom Plantae or "Viridiplantae") in terms of the numbers of described species. Approximately 260,000 species of flowering plant have been named so far -- constituting nearly 90% of all known species of plants. Even so, taxonomists describe many new species annually, and estimates of total angiosperm diversity reach as high as 400,000 species. There are about 450 families of flowering plants, and they display extremely diverse life-histories and ecological adaptations. In addition to dominating plant biodiversity, angiosperms are the dominant photosynthesis/photosynthetic organisms (primary producers) in most terrestrial ecosystems (an important exception to this rule are the boreal forests (Forest biome), which are often dominated by conifers). All important food plants are angiosperms.
Figure 1. Diversity in the shapes and forms of vegetative and reproductive parts across different angiospermic families Credits: Saikat Basu Source: Saikat Basu, own work
Natural history
Angiosperms are also the youngest of the plant divisions, having arisen relatively late in the history of terrestrial plant life. The first land plants are about 450 million years old, but the earliest definitive angiosperm fossils are only about 130 million years old, placing their known origins within the Early Cretaceous. However, indirect evidence leads some scientists to estimate that angiosperms may have originated as early as 250 million years ago, i.e. at the end of the Permian.
Figure 2. Different types of inflorescence (floral arrangement) among angiosperms Credits: Saikat Basu Source: Saikat Basu, own work
By about 100 million years ago, during the LateCretaceous, angiosperms had experienced a rapid diversification in forms. The factors driving this diversification have long been of interest to evolutionary biologists. Many scientists hypothesize that specialized mutualistic relationships with animal pollinators played an important role in the development of angiosperm diversity (and perhaps insect diversity as well).
Figure 3. Different foliage types and arrangements among angiosperms Credits: Saikat Basu Source: Saikat Basu, own work
Morphology
Angiosperms are anatomically distinguished from other plant groups by several developmental and anatomical features:-
- They bear flowers, which are very short branches bearing a series of closely-spaced leaves modified to facilitate pollination (sepals and petals) or to bear the organs involved in sexual reproduction (stamens and pistils);
- Developing seeds are completely enclosed in an ovary derived from a portion of the pistil (the word angiosperm is of Greek derivation, meaning covered seed);
- Ovary tissues mature to form a fruit that is generally involved in protecting the seed and facilitating its dispersal (only angiosperms bear true fruits);
- Seeds, at some point in their development, contain a distinctive tissue, the triploid endosperm, which serves as a nutritional reserve for the developing embryo.
Figure 4. Diversity in pollen types among angiosperms. A. Trigonella foenum-graecum L. (Fabaceae); and B. Brassica napus L. (Brassicaceae) Credits: Saikat Basu Source: Saikat Basu, own work
Notable families
Some of the most important and diverse flowering plant families are:
- the composite or daisy family (Asteraceae) which, with about 24,000 named species, may be the largest plant family;
- the orchid family (Orchidaceae) which rivals the daisy family in diversity, with about 17,000 species named;
- the grass family (Poaceae) with over 10,000 species, includes threeof the four most productive human food-plants -- rice (Oryza), wheat (Triticum) andmaize (Zea): the fourth, the potato (Solanum tuberosum), isa member ofthe nightshade family (Solanaceae).
- the bean or legume family (Fabaceae, also known as Papilionaceae),which includesabout 19,000 species: many species are important in human food because their symbiotic association with certain nitrogen-fixing bacteria leads to unusually high protein content.
Figure 5. Diversity of plant habits across angiospermic families Credits: Saikat Basu Source: Saikat Basu, own work
Figure 6. Diversity of fruits among angiosperms (Source: Saikat Basu, own work)
Figure 7. Economic importance of angiosperms. A-C. Crop production under greenhouse conditions (A. Fenugreek, B. Cereals. and C. Tomato); D-I. Field Crops (D. Cereal-Wheat; E. Forage-Alfalfa; F. Legume-Fenugreek; G. Beverage-Tea; H. Spice-Fennel; I-Vegetable-Lettuce); J-L. Weeds (J. Weed infestation in the filed and I. Greenhouse; L. Weed infestations at the edge of agricultural lands). (Source: Saikat Basu. own work).
References
- Basinger, J., and D. L. Dilcher. 1984. Ancient bisexual flowers. Science 224: 511-513.
- Beck, C. B., ed. 1976. Origin and Early Evolution of Angiosperms. Columbia University Press, New York.
- Behnke, H.-D. 1969. Die Siebrohren-Plastiden bei Monocotlen. Naturwissenschaften 55: 140-141.
- Bell, C. D., D. E. Soltis, and P. S. Soltis. 2005. The age of the Angiosperms: a molecular timescale without a clock. Evolution, 59(6).
- Bharathan, G., and E. A. Zimmer. 1995. Early branching events in monocotyledons–partial 18S ribosomal DNA sequence analysis. In P. J. Rudall, P. J. Cribb, D. F. Cutler, and C. J. Humphries [eds.], Monocotyledons: systematics and evolution, 81-107. Royal Botanic Gardens, Kew, London, UK.
- Borsch, T., K. W. Hilu, D. Quandt, V. Wilde, C. Neinhuis, and W. Barthlott. 2003. Non-coding plastid trnT-trnF sequences reveal a highly supported phylogeny of basal angiosperms. Journal of Evolutionary Biology 15: 558-567.
- Gottsberger, G. 1988. The reproductive biology of primitive angiosperms. Taxon 37: 630-643.
- Graham, S. W., P. A. Reeves, A. C. E. Burns, and R. G. Olmstead. 2000. Microstructural changes in noncoding chloroplast DNA: interpretation, evolution, and utility of indels and inversions in basal angiosperm phylogenetic inference. International Journal of Plant Sciences 161 (Supplement): S83-S96.
- Graham, S. W., and R. G. Olmstead. 2000. Utility of 17 chloroplast genes for inferring the phylogeny of the basal angiosperms. American Journal of Botany 87: 1712-1730.
- Heckman, D. S., D. M. Geiser, B. R. Eidell, R. L. Stauffer, N. L. Kardos, and S. B. Hedges. 2001. Molecular evidence for the early colonization of land by fungi and plants. Science 293: 1129-1133.
- Heywood, V. 1993. Flowering plants of the world. B.T. Batsford Ltd., London, UK.
- Hickey, L. J., and A. D. Wolfe. 1975. The bases of angiosperm phylogeny: vegetative morphology. Annals of the Missouri Botanical Garden 62: 538-589.
- Hillis, D. M. 1996. Inferring complex phylogenies. Nature 383: 130.
- Nandi, O. I., M. W. Chase, and P. K. Endress. 1998. A combined cladistic analysis of angiosperms using rbcL and nonmolecular data sets. Annals of the Missouri Botanical Garden 85: 137 - 212.
- Nickerson, J., and G. Drouin. 2004. The sequence of the largest subunit of RNA polymerase II is a useful marker for inferring seed plant phylogeny. Molecular Phylogenetics and Evolution 31: 403-415.
- Nickrent, D. L. and D. E. Soltis. 1995. A comparison of angiosperm phylogenies from nuclear 18S rDNA and rbcL sequences. Annals of the Missouri Botanical Garden 82:208-234. Parkinson, C. L., K. L. Adams, and J. D. Palmer. 1999. Multigene analyses identify the three earliest lineages of extant flowering plants. Current Biology 9: 1485-1488.
- Nickrent, D. L., A. Blarer, Y.-L. Qiu, D. E. Soltis, P. S. Soltis, and M. Zanis. 2002. Molecular data place Hydnoraceae with Aristolochiaceae. American Journal of Botany 89: 1809-1817.
